Each pixel in such liquid crystal devices acts as an independent rotatable waveplate, such that an applied voltage rotates the birefringent axes through an angle 2.theta.. Until now, the accepted mode of operation of such devices has been to arrange and correctly orient crossed polarisers around the device. Liquid crystal spatial light modulators may be configured as binary phase computer generated holograms (CGHs) which have particular application to free space diffraction based optical switching. This is described in detail in an article entitled "A holographically routed crossbar: Theory and simulation" by D. C. O'Brien, W. A. Crossland, and R. J. Mears, published in Optical Computing and processing, 1(3): 233-243, 1991. In this case, the ideal transmission of the device through crossed polarisers can be shown to be EQU T+sin.sup.2 2.theta.sink .DELTA.nd/2 (1)
where k is the wave number 2.pi./.lambda., .DELTA.n is the FLC birefringence and d is the FLC thickness. This is described in an article entitled "Programmable binary phase-only optical device based on ferroelectric liquid crystal SLM" by S. E. Broomfield, M. A. A. Neil, E. G. S. Paige, and G. G. Yang, published in Electronics Letters, 28(1):26-28, 1992. Unfortunately the intrinsic polariation sensitivity severely limits the use of such switches in normal optical fibre systems where polarisation is usually not preserved.
A special situation occurs provided the device is manufactured with half waveplate thickness and an aggregate electro-optic switching angle of 2.theta.=90.degree.. This is described in an article entitled "Diffractive ferro-electric liquid crystal shutters for unpolarized light", by M. J. O'Callaghan and M. A. Handschy, published in Optics Letters, 16(10):770-772, 1991. However few large angle materials exist and they are difficult to prepare; the alternate approach uses two layers of 45.degree. material and this is described in an article entitled "Improved transmission in a two level, phase only, spatial light modulator", by M. A. A. Neil and E. G. S. Paige, published in Electronics Letters, 30(5):445-446, 1994.